Wireless local area networks (LANs) have been used for the past several years to untether data applications in the local area. In addition, wireless LANs are now being proposed for supporting real-time voice and video applications, even though the characteristics and performance requirements of data and real-time traffic differ substantially. Real-time traffic requires bounded end-to-end delays beyond which information loses its value and may be discarded. In contrast, the end-to-end delay requirements of data traffic are less stringent. The Institute of Electrical and Electronics Engineers (IEEE) formed a study group that formulated IEEE standard 802.11, which supports real-time traffic over a wireless LAN.
IEEE standard 802.11 supports real-time traffic over a wireless LAN by switching from a normal, distributed access mode to a centralized mode. The standard recommends that the medium access control (MAC) layer use a carrier sense multiple access/collision avoidance (CSMA/CA) scheme as the basic access control mechanism. This scheme is a variant of the well-known CSMA with collision detection (CSMA/CD) scheme, which is used in hard-wired LANs and which has now been adapted to meet the constraints of the wireless environment. The CSMA/CD scheme has long been recognized as unsuitable for supporting real-time applications.
Supporting real-time traffic is likely to be even more problematic in wireless LANs. Current wireless LANs typically operate at 2 Mb/s, whereas hard-wired LANs that use CSMA/CD typically operate at 10 Mb/s. Therefore, for the same relative loads, wireless LANs have higher queuing delays. For comparable system parameters, the CSMA/CA protocol has even worse throughput-delay characteristics than the CSMA/CD protocol.
While different segments of a hard-wired LAN generally are electromagnetically isolated, neighboring cells of a wireless LAN often interfere with each other, thereby decreasing the individual throughputs of the cells. Moreover, the most effective solutions for providing priority to real-time traffic in CSMA/CD-based LANs rely on the collision detection ability of hard-wired transceivers, which are not utilized in wireless communications. In an effort to provide a more suitable solution for supporting real-time traffic than the CSMA/CA protocol, the MAC layer of the IEEE 802.11 provides for an optional centralized access mode that is based on polling rather than carrier detection. However, the use of this centralized scheme imposes heavy constraints on the operation of wireless LANs. First of all, the centralized mode cannot be operated simultaneously in neighboring cells. Secondly, the centralized mode requires the existence of an access point (AP) with specialized access functions. In addition, it has been determined that the centralized scheme of the IEEE 802.11 standard results in poor performance.
In an article entitled “Real-Time Traffic Over the IEEE 802.11 Medium Access Control Layer,” by Sobrinho and Krishnakumar, a modification to the IEEE 802.11 standard is discussed that guarantees bounded access delays to real-time traffic and which does not rely on collision detection capability. The real-time access points (APs) provide real-time traffic with priority over the data APs to ensure that real-time traffic is provided with a guaranteed bandwidth. The medium access control protocol discussed in the article is intended to govern communication between APs and wireless devices such as, for example, cellular telephones and laptop computers. Real-time traffic is communicated to real-time APs and data is communicated to data APs. The APs then communicate the real-time information and data to the CN serving the APs.
The MAC layer proposed in the aforementioned article utilizes a technique known as Black bursts to enable real-time APs to organize themselves in such a way that each accesses the channel between the wireless devices and the APs at regularly-spaced and distinct time intervals with minimal disruption to surrounding data traffic. As APs enter and leave the network, the Black bursts technique provides the mechanism by which access instants are re-aligned to accommodate the change in traffic patterns while preserving the basic rules for fair medium access.
It would be advantageous to provide a medium access control protocol that would provide similar advantages for communications between APs and CNs in a backhaul network configuration. In a typical backhaul configuration, the CNs comprising points of entry into a wired network collect data and/or real-time information from the APs that they serve and then the data and/or real-time information is transported, or backhauled, over a wired link, such as an optical fiber trunk. It would be advantageous to adapt the Black bursts technique to a backhaul network configuration to enable access instants in communicating between APs and CNs to be re-aligned in response to changes in the network (e.g., APs entering and leaving the network) without requiring the APs to explicitly inform each other of the changes. This is an especially desirable capability in the quest to enable wireless networks to become entirely self-configuring. The present invention meets these and other objectives.